1. Field of the Invention
The present invention relates to a wireless integrated circuit (IC) device and to a method of manufacturing the wireless IC device. More specifically, the present invention relates to a wireless IC device including a wireless IC that is used in radio frequency identification (RFID) systems and to a method of manufacturing the wireless IC device.
2. Description of the Related Art
A non-contact IC card described in Japanese Unexamined Patent Application Publication No. 2001-10264 is an example of a known wireless IC devices used for access management, commuter passes, credit cards and other applications. FIG. 19A is a top surface view of a non-contact IC card 100 described in Japanese Unexamined Patent Application Publication No. 2001-10264 and FIG. 19B is bottom surface view of the non-contact IC card 100 described in Japanese Unexamined Patent Application Publication No. 2001-10264.
In the non-contact IC card 100 illustrated in FIGS. 19A and 19B, an antenna coil 104, which winds in a spiral shape a plurality of times, is arranged on a main surface of a substrate 102, and an adjustment resistor (not illustrated in FIGS. 19A and 19B) and an adjustment capacitor 108 are connected to the antenna coil 104. Furthermore, an IC 106 is connected to the antenna coil 104. In this non-contact IC card, by trimming a portion of the adjustment resistor and the adjustment capacitor 108 during manufacturing, the resistance value and the capacitance value of the non-contact IC card 100 can be adjusted and the resonant frequency and sharpness (Q) can be adjusted.
However, with the non-contact IC card 100, as will be described below with reference to the drawings, the inventors of the present invention discovered that the resonant frequency varies during use. FIG. 20A is a sectional structural view of the antenna coil and the substrate of the non-contact IC card 100 taken along line B-B and FIG. 20B is an equivalent circuit diagram of the non-contact IC card 100. The substrate 102 and the antenna coil 104 are illustrated in FIG. 20A. In addition, in FIG. 20B, an inductance L100 of the antenna coil 104, a resistance R100 of the IC 106 and a capacitance C100 of the antenna coil 104 are illustrated.
In the non-contact IC card 100, the antenna coil 104 winds a plurality of times in a spiral shape on the main surface of the substrate 102. In the non-contact IC card 100, as illustrated in FIG. 20A, the wires that define the antenna coil 104 are arranged side by side and close to each other on the main surface. When a current flows through the wires arranged close to one another, due to the potential difference between the wires, electric force lines E100 are generated that link the wires, as illustrated by the arrows in FIG. 20B, and the capacitance C100 is generated between the wires. The capacitance C100, as illustrated in FIG. 20B, is connected in parallel with and between the inductance L100 and the resistance R100. Furthermore, in the non-contact IC card 100, the shape of the antenna coil 104 is designed so that desired values of the inductance L100 and the capacitance C100 are obtained that result in a desired resonant frequency.
However, even when the shape of the antenna coil 104 is designed so that the desired resonant frequency can be obtained, the inventors of the present invention discovered that the resonant frequency of the non-contact IC card 100 varies during use. Consequently, the inventors of the present invention performed experiments and computer simulations and investigated the cause of the variations of the resonant frequency of the non-contact IC card 100. As a result, the occurrence of a phenomenon described below in the non-contact IC card 100 was determined to be the cause of the variations of the resonant frequency.
The non-contact IC card 100 is used, for example, for access management or as a commuter pass or a credit card. This type of non-contact IC card 100 is usually used by being moved close to a dedicated reader/writer while being held in a person's hand. Therefore, as illustrated in FIG. 20A, during use, the person's finger is located in the vicinity of the antenna coil 104 and the electric force lines E100 pass through the person's finger. Since the dielectric constant of a person's finger is much greater than that of air, when the person's finger is moved close to the space between the wires of the antenna coil 104, the capacitance C100 generated between the wires of the antenna coil 104 is increased. As a result, the resonant frequency of the non-contact IC card 100 is reduced to less than the desired resonant frequency.
The manner in which the non-contact IC card 100 is held often differs during use, and therefore, the positional relationship between the wires of the antenna coil 104 and the person's hand is not fixed. Accordingly, the amount by which the capacitance C100 increases also varies during use and the amount by which the resonant frequency of the non-contact IC card 100 decreases also varies during use. In other words, the resonant frequency of the non-contact IC card 100 varies during use. Since the resonant frequency of the non-contact IC card 100 varies during use, the desired resonant frequency cannot be obtained by trimming an adjustment capacitor during manufacturing to adjust the resonant frequency.